Logs provide detail that metrics cannot. But they're expensive to store and query.
Common stacks:
- ELK: Elasticsearch, Logstash, Kibana. Powerful but resource-heavy
- Loki: Grafana's log aggregator. Indexes labels, not content. Cheaper
- Cloud-native: CloudWatch Logs, Cloud Logging
Structured logging:
{"timestamp": "2024-01-15T10:30:00Z", "level": "error", "service": "auth", "user_id": "12345", "message": "login failed"}
Benefits: Searchable fields, consistent format, easier parsing.
Interview tip: Always recommend structured logging. It costs the same but is far more useful for debugging.